Technical Field
The present invention relates to a highly flexible copper-clad laminate wherein a copper foil is laminated onto a polyimide film.
Background Art
A polyimide film has been used for a laminated sheet, a flexible printed-circuit board and so forth, because of its excellent heat resistance, chemical resistance, mechanical strength, electric properties and so on.
For example, a commonly used flexible printed circuit board (FPC) is a copper-clad laminate wherein a copper foil is laminated on one side or both sides of a polyimide film (Patent documents 1-3). A polyimide film used generally has a thickness of about 25 μm as the Examples in Patent documents 1-3.
At present, a copper-clad laminate, typically a substrate for electronic parts, requires excellent mechanical properties and higher flexibility. In particular, when a copper-clad laminate is applied to a hinge, it is needed to be highly flexible. However, a conventional copper-clad laminate comprising a polyimide film with a thickness of about 25 μm may not be adequately flexible.
Furthermore, a polyimide film may not have sufficiently adhesive properties. For improving its adhesiveness, it undergoes surface processing such as alkaline treatment, corona treatment, sandblasting and low-temperature plasma treatment, for example. Although being effective for improving adhesiveness, however, these methods require the use of an adhesive other than a polyimide, for example, an epoxy resin adhesive, leading to deterioration in heat resistance of the flexible substrate as a whole.
Thus, there has been proposed, as a polyimide film, a thermocompression-bonding multilayer polyimide film wherein thin layers of a thermoplastic polyimide are laminated on both sides of a heat-resistant polyimide layer.
However, when this thermocompression-bonding multilayer polyimide film has a smooth surface, it has a larger friction against a roll, for example, during film formation in which the film is winded onto a wind-up roll or during lamination with a copper foil, causing troubles such as wrinkle formation and twisting around a roll, which may restrict winding. Accordingly, there are needs to improve the surface slipping property of a polyimide film.
Examples of a method used for improving surface slipping property of a polyimide film include surface treatment such as embossing, and methods to reduce a surface friction coefficient by dispersing an inorganic powder such as calcium phosphate (Patent document 1) and silica (Patent document 2) in a polyimide film to form minute protrusions on the film surface. Alternatively, there has been proposed a method for manufacturing a polyimide film, by flow casting, from a solution of a polyamic acid polymerized in a solvent in which a fine inorganic filler is dispersed (Patent document 3).
However, the first method of surface treatment has a drawback that the appearance of the film is apt to be impaired due to excessive roughness formed of the film surface. In the second method wherein an inorganic powder is mixed with a solution of a polyamic acid to prepare a polyimide film, it is difficult to disperse the inorganic powder in the polyamic acid solution homogeneously, without using a special dispersing device. Thus, in this method, undispersed inorganic powder may remain as a lump, and thereby a prominent protrusion may be formed on the surface of the film obtained. In the third method, similarly, it is difficult to disperse a fine-particulate inorganic powder homogeneously, and the use of an inorganic powder with a larger particle size may lead to the same problem as that encountered in the second method.
Therefore, when these methods in which inorganic fillers are added are applied to a copper-clad laminate for a COF requiring a fine pattern, a protrusion on a thermoplastic polyimide surface may hinder the formation of fine pitches.